1. Field of the Invention
This invention relates generally to an apparatus for recording and reproducing a video signal, and more particularly, to an improvement of a recording circuit and a reproducing circuit.
2. Description of the Prior Art
In a known video tape recorder (VTR), a luminance signal is recorded in the form of an FM signal. In a video signal according to the so-called high definition vision system, the luminance signal has a bandwidth of 10 MHz. If this luminance signal of 10 MHz is frequency modulated on a carrier having the frequency of, for example, 18 MHz, then second and third side band components thereof caused by this modulation are folded with respect to the center frequency so as to fall within a band of a first side band component. Accordingly, if the FM signal is demodulated under such a state, there is produced a moire component by the folded portions, resulting in deterioration of the image quality.
FIG. 1A shows the folded state wherein, J.sub.0 represents a carrier, J.sub..+-.1 first side band components, J.sub..+-.2 second side band components and J.sub..+-.3 third side band components. In this case, the upper and lower side bands of the same side band components are equal to each other in level, but their phases are inverted.
In order to suppress the occurrence of a moire component, the assignee of the present application has previously proposed (see Japanese patent application No. 61-282671) that the phase of the undesired side band components, for example, the upper side band J.sub.+2 or the or the lower side band J.sub.-2 of the second side band components J.sub..+-.2, be rotated to remove the second side band components J.sub..+-.2.
It appears that the above-mentioned proposal is effective because the side band components of the FM signal in which the phases of the upper and lower side bands of the same side band components are inverted relative to each other, as shown in FIG. 2A, are not removed by a limiter, as shown in FIG. 2B. On the other hand, the side band components of the AM signal in which the phases of the upper and lower side bands of the same side band components are equal, as shown in FIG. 3A, can be removed by the limiter, as shown in FIG. 3B.
According to the above-mentioned proposal in which the phase of only the upper side band or the lower side band of the undesired side band components is rotated, it is possible to suppress only the moire component caused by a specific narrow band, for example, the second side band components J.sub..+-.2. The problem then is that, when the phase of the side band in the specific narrow band is rotated, a direct current component occurs in a reproduced signal. This direct current voltage (DC voltage) component results in distortion of the reproduced image.
A defect of a prior art reproducing circuit will now be explained with reference to FIG. 4 in which an FM video signal is reproduced from a magnetic tape 1 by a magnetic reproducing head 2 and is supplied through a playback amplifier 3 to an equalizer 4. In the reproduced FM signal supplied to the equalizer 4, the level of lower side band component J.sub.-1 is substantially as high as that of carrier J.sub.0 as shown in FIG. 5A. The condition J.sub.0 .ltoreq.J.sub.-1 .times.2 causes a so-called overmodulation, which causes the equalizer 4 to attenuate the level of the lower side band component J.sub.-1 as shown in FIG. 5B. In FIGS. 5A and 5B, J.sub.+1 represents the upper side band component and the hatched portion N represents the noise. When the equalizer 4 attenuates the level of the lower side band component J.sub.-1, the level of the noise N on the lower side band is attenuated accordingly.
Referring back to FIG. 4, the output signal of the equalizer 4 is supplied through a limiter 5 to an FM-demodulator 6, and a reproduced video signal is developed at an output terminal 7 that is supplied from the output side of the FM-demodulator 6.
The noise N contained in the reproduced video signal has a triangular characteristic, that is, the level is increased in proportion to the frequency, as shown by the hatched portion in FIG. 5C. However, the noise level N of the lower side band component J.sub.-1 is attenuated by the equalizer 4 as mentioned above, and is shown by a dashed line in FIG. 5C
Various proposals have been advanced for improving the S/N ratio by utilizing the lower side band more efficiently. One of such proposals is to utilize playback network circuit in which a low-pass filter with a linearly-decreasing frequency characteristic is used to process an FM signal at a uniform constant phase, thereby utilizing the lower side band more efficiently. Another proposal is to use a double limiter in which an FM signal is limited by a band-pass filter having a carrier frequency as a center frequency and then added to a lower side band component to effect the modulation in the high band, thereby utilizing the lower side band efficiently.
Accordingly, the previously mentioned proposals can improve the S/N ratio by utilizing the lower side band more efficiently; however, the improvement of the S/N ration is limited by the use of the lower side band component, and a distortion is also produced.